As air quality standards become more stringent, considerable efforts have focused on minimizing the particulate matter emitted in diesel engine exhaust. A potential solution is a particulate trap inserted in the exhaust system of a diesel engine.
A honeycomb ceramic wall-flow through filter, such as described by U.S. Pat. Nos. 4,276,071; 4,329,162 and 4,857,089; and 5,098,455, has become the preferred type of particulate trap. These honeycomb filters are made by extruding a paste comprised of water, binder and ceramic powders (for example, clay, talc, mullite, silica, silicon carbide and alumina) that form, for example, cordierite upon firing. Generally, the materials of choice for Diesel particulate filters have been cordierite, silicon carbide and mullite. Each of these, however, suffers from one or more problems.
In making ceramic honeycombs, clays, water soluble binders or combinations thereof are generally used to make the paste sufficiently plastic to form extruded useable honeycombs. After the paste is extruded, the honeycomb is dried, debindered and sintered to form a honeycomb. The honeycomb is heated to sinter or fused together the ceramic particulates or grains.
Even though cordierite has excellent thermal shock resistance due to its low thermal expansion coefficient, it suffers from a low use temperature, which may be exceeded when removing soot by combustion during the operation of a Diesel engine. In addition, cordierite because it is a result of a sintering process has tortuous porosity that leads to high pressure drops and as such limits the loading that can occur.
Silicon carbide, on the other hand, has good strength and high temperature resistance, but also suffers from high pressure drops due to similar porosity as cordierite. Silicon carbide also requires for it to have the good high temperature properties to be made without clay binders and as such are difficult to make into large parts requiring smaller extruded parts to be segmented and then assembled into larger filters. Lastly, silicon carbide even though it has reasonably good thermal shock resistance when the rate of temperature change is not too great due to its good thermal conductivity, still is prone to failure under rapid temperature changes due to its higher thermal expansion coefficient.
Mullite even though it has demonstrated good high temperature resistance, low pressure drops, high soot capture efficiency, it too has a high thermal expansion coefficient, which may be problematic in certain applications.
Accordingly, it would be desirable to provide a method for making wall-flow traps, for example, that avoids one or more of the problems of the prior art, such as one of those described above.